1. Field of the Invention
The present invention relates to a glass antenna for a vehicle in which an antenna conductor and first and second feeding portions adjacent to each other are provided on window glass. In addition, the invention relates to window glass for a vehicle including the glass antenna for a vehicle.
2. Description of the Related Art
As a means for solving a fluctuation (fading) in the receiving level of a radio wave caused by interference between a direct wave and a reflected wave reflected by an obstacle, such as a mountain or a building, for example, a diversity method disclosed in JP-A-6-21711 is known in the related art. The antenna device for a vehicle disclosed in JP-A-6-21711 includes a main antenna, which receives an FM broadcast and outputs an FM main signal, and a sub-antenna, which receives an FM broadcast and outputs an FM sub-signal, on rear window of the vehicle. The FM main signal and the FM sub-signal are mixed in a state having a predetermined phase difference therebetween, and the phase difference is changed so that a sufficient signal level is obtained in signal reception when the mixed level is lower than a predetermined value. That is, the mixed level is changed by adjusting the phase difference at the time of mixing.
In general, received signals of radio waves received through a plurality of antennas become theoretically uncorrelated with each other by ensuring the spatial distance between the antennas according to the wavelength of a radio wave to be received. As a result, it is known that a so-called space diversity effect is obtained. That is, since a correlation coefficient indicating the degree of correlation between an amplitude fluctuation in a received wave received through one antenna and an amplitude fluctuation in a received wave received through another antenna can be reduced according to an increase in the distance between a plurality of antennas, the space diversity effect can be sufficiently realized.
In the case of a glass antenna formed on window glass, however, it is not possible to measure a physical distance between antennas unlike a pole antenna. For this reason, antenna design based on the spatial distance was difficult. Therefore, the inventor of this application discovered that in the case of a glass antenna in which two antenna conductors were provided on window glass for a vehicle, a sufficient space diversity effect could be realized on the glass antenna if a phase difference δ between a received wave received through one antenna conductor and a received wave received through the other antenna conductor was large when a radio wave with a fixed frequency was transmitted. That is, the phase difference δ and the distance between antennas can be considered to be equivalent to each other.
Accordingly, in order to obtain the sufficient space diversity effect demanded, it is necessary to increase the phase difference δ detected as a characteristic of a glass antenna itself by tuning the arrangement position of an antenna conductor, the shape of the antenna conductor itself, and the like. For example, if the arrangement positions of feeding portions of two antenna conductors are distant from each other, the arrangement positions of the two antenna conductors also tend to be distant from each other. In this case, the phase difference δ can be easily increased.
However, depending on the specifications of a vehicle, such as the setting position of a feeding portion or a wiring location, feeding portions of two antenna conductors may have to be brought close to each other. In this case, it is not easy to increase the phase difference δ.